THE PHYLUM PLATYHELMINTHES 



209 



Nucleus 



Ciliary 

 "flame" 



-Tubule 



'^/ • M tubule 



fxWm 





-plaraz, 

 cell 



A B 



Figure 1 1 .6. Excretory system of Dugesia. A, Detail showing flame cells and tubules. 

 B, The tubule network. 



Metabolic wastes other than water are believed to pass from the 

 body simj)ly by diffusion. 



76. Dugesf'a: Reproduction 



Throughout most of the year no reproductive organs are evident 

 in Dugesia. If an individual is well fed, it grows and reproduces asex- 

 ually by pulling itself into two pieces. The body becomes elongated 

 posterior to the pharynx, then this region becomes stretched, attenuated, 

 and finally ruptures. The anterior end moves off and in about one day a 

 new tail begins to form. If it continues to be well fed, the process can 

 be repeated. The posterior end rounds up and becomes quiescent. In 

 a few days it will grow a head and pharynx. At first it is very small, 

 but with feeding it soon becomes full size and may itself reproduce 

 asexually. 



In the spring a reproductive system develops from the mesenchyme 

 in most populations. Each individual is hermaphroditic, having com- 

 plete sets of male and female organs for the production, storage and 

 transfer of the sex cells. When sexually mature, pairs copulate fre- 

 quently. The initial step or "courtship" involves a series of repeated 

 head and body contacts, obviously different from the casual way in 

 which sexually undeveloped individuals pass by each other. The two 

 individuals gradually assume a copulatory position, facing somewhat 



